This invention is in the field of electronic power supplies. In particular, it relates to microprocessor controlled power supplies for battery powered personal computers.
Portable computers are one of the fastest growing segments of the computer market. As the capabilities of these machines have increased, and as their size has decreased, the advantages of computer technology have become almost universally available. Although such portable computers can be used in a fixed base mode of operation, during which operation they are connected to a conventional, A.C. power supply, it is at least equally likely that these portable computers will be used in places where A.C. power is not available, such as within an airplane. During use in such environments, portable computers rely on their internal battery packs for power.
Sophisticated battery packs and related power supplies are an absolute necessity for portable computers. A portable computer must be able to operate continuously for a reasonable amount of time with a fully charged battery pack. Additionally, sudden power failures, with their consequent loss of data, must be kept to an unavoidable minimum. As a matter of course, the battery pack and power supply should together provide some indication to the user that power is about to be lost, so that data can be stored and a graceful shut-down effected.
Unfortunately, many new types of battery packs used in portable computers make the task of determining loss-of-power events very difficult. As shown in FIG. 1, the voltage output of some battery packs declines relatively linearly with use, with the battery pack's voltage level dropping towards the discharge voltage toward the end of the battery pack's charge. At some point, the battery pack will not provide enough capacity for the computer to operate. If this point is reached without warning, data will be lost.
Determining the warning point when the power supply must signal the user to save data prior to final loss-of-power is not a simple task. Battery discharge can be affected by such factors as temperature, battery type, the load on the battery, the battery's age, and the recharge method and number of recharge cycles that the battery has undergone. All these factors result in a non-linear voltage output versus time of use curve for the battery pack, which non-linear curve is much more typical of battery pack performance than the more nearly linear performance of sealed lead acid batteries illustrated in FIG. 1. A particular example of this is shown in FIG. 2, which illustrates the time of use versus voltage output curve of a nickel-cadmium (`NiCad`) battery pack. As shown in the figure, this type of battery pack continues to output at the same, high level (roughly within 5% of the original output level) for a very large percentage of its output time. However, once a certain point is reached, the NiCad's voltage output drops very rapidly towards zero. With such a battery, it is quite difficult to determine when proper power-down warning should be given to the user.
To date, most manufacturers have dealt with the problem of warning of loss-of-power by simply making a very conservative estimate of the total capacity in a fully charged battery pack and then signaling a low-power warning at a time when it is certain that sufficient power remains in the battery pack to achieve a successful shut-down. This conservative estimate not only forces the user to either stop using the computer or change batteries sooner than would ideally be necessary, but can also affect future charge-discharge patterns of the battery, as the recharging process begins with only a partially discharged battery.
There are many additional shortcomings in known portable computer power supplies. Most such circuits can only function with one specific type of battery pack. Modification of the supply to deal with other types of battery packs is either impossible or very difficult. Further, such circuits only make a crude estimate of the total charge in a given battery pack and cannot provide an optimum recharge to the battery pack. Likewise, as the supplies do not compensate for such factors as temperature or current drain, their estimates of remaining battery life and proper recharge can be very wrong. In some cases, the estimate is sufficiently wrong that even the conservative estimate made to signal "power off" is incorrect and results in a failure to provide adequate warning of loss-of-power and lost data.
A power supply capable of making more precise power measurements and providing optimum recharge currents, compensating properly for variations in environmental factors, would be of great advantage to the portable computer industry.